The present invention relates to a speech processing apparatus and, more particularly, to a linear predictive type speech analysis and synthesis apparatus capable of lowering the bit rate and improving synthesized speech quality by making use of multi-pulses as its speech information.
The vocoder can encode a speech signal within a very narrow bandwidth in which a linear prediction coefficient (called "LPC coefficient") as a spectrum envelope parameter, and exciting source information, including a voice/unvoiced discrimating signal, are transmitted from an analysis side to a synthesis side, and a synthesized speech signal is obtained by using a digital synthesis filter having filter coefficients determined by the LPC coefficients and driven by the exciting source signal.
Such a vocoder can encode a speech signal within a very narrow bandwidth at a low bit rate of 1,200 to 2,400 bps (i.e., bit per second). However, these conventional vocoders have a problem of poor synthesized speech quality due to the simplicity of the speech generation model and the difficulty in an accurate pitch extraction.
To solve the above problem, there has been proposed a multi-pulse vocoder. A vocoder of this type expresses the exciting source information by a plurality of pulses, i.e., multi-pulses regardless of whether the speech is voiced or unvoiced, to utilize the waveform information of the speech signal so that the synthesized speech quality is remarkably improved. This type of vocoder, on the other hand, causes another problem of a increase in coding rate (bit rate).